Nickel free austenitic stainless steels

ABSTRACT

Austenitic stainless steels containing 0.06 to 0.15% of C, 0,3 to 1.0% of Si, 13 to 16% of Cr, 7.0 to 12.0% of Mn, 0.05 to 0.15% of N and 1.0 to 4.0% of Cu and the balance of Fe as the essential ingredients and assuming a fully austenitic phase after annealing.

United States Patent [191 Hoshino et al.

NICKEL FREE AUSTENITIC STAINLESS STEELS Inventors: Kazuo Hoshino; Koutaro Morita,

both of Yamaguchi, Japan Assignee: Nisshin Steel Co., Ltd., Tokyo,

Japan Filed: Mar. 26, 1973 Appl. No.: 344,509

Related U.S. Application Data Continuation of Ser. No. 97,494, Dec. 14, 1970, abandoned.

Foreign Application Priority Data Apr. 30, 1970 Japan 45-36324 US. Cl. 75/125; 75/126 B; 75/126 J Int. CL C220 39/26; C22C 39/54 Field of Search... 75/125, 126 A, 126 B, 126 J 51 July 8,1975

[56] References Cited UNlTED STATES PATENTS 2,3l0.308 2/1943 Morrison 75/125 2,862,812 12/1958 Dulis 75/126 B 2,876,096 3/1959 Payson 75/126 B 3,075,839 l/l963 Dulis 75/126 8 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmFinnegan, Henderson, F arabow and Garrett 7 Claims, 2 Drawing Figures WHWNUL 8 m5 3.893850 SHEET 1 INVENTORS KAZ UO HOSHINO KOUTARO MORITA Z2129 an, fi e/202x500 fazafiaw ATTORNEYS NICKEL FREE AUSTENITIC STAINLESS STEELS This application is a continuation of application Ser. No. 97,494 filed Dec. l4, 1970, now abandoned.

The present invention relates to austenitic stainless steels free of Ni, particularly to those remarkably improved in formability and corrosion resistance, characterized by containing as the essential ingredients 0.06 to 0.15% of C, 0.3 to l.0% of Si, l3 to 16% ofCr, 7.0 to 12.0% of Mn, 0.05 to O.l5% of N and L0 to 4.0% of Cu and the assumption of the fully austenitic phase after annealing.

In general, stainless steels can be classified roughly into the 13% Cr martensitic stainless steels, the 18% Cr ferritic stainless steels and the IS% Cr, 8% Ni austenitic stainless steels. The 13% Cr martensitic stainless steels are high-strength materials generally used for structural and cutlery-manufacturing purpose, and obtained by a martensitic transformation which occurs at the transformation point AC at high temperature by controlling the diffusion transformation of the austenitic phase. AlSl types 410 and 420 are classified into this class. The stainless steels of this class are characterized by such disadvantages as the inferior corrosion resistance because of the low Cr content and the occurrence of cracks at the time of welding because of martensitic transformation.

The l 8% Cr ferritic stainless steels are made by transforming the bainitic structure into the ferritic structure by means of diffusion annealing after hot-rolling, and they are used quite extensively. However, this class also is inferior to the l8% Cr, 8% Ni austenitic stainless steels with respect to corrosion resistance, formability, particularly stretch-formability and weidability. This class is exemplified by AIS] type 430. Further, the 18% Cr, 8% Ni austenitic stainless steels are steels which retain the austenitic phase, which exists at high temperature in the 18% Cr ferritic class, at room temperature by adding a large amount of Ni. Generally speaking, they are superior to the IS% Cr ferritic stainless steels in mechanical properties, (ductility and toughness), formability, weldability and corrosion resistance. However, Ni is very expensive and thus presents a very serious drawback to its use in this field.

AISI types 201 and 202 have recently been developed, wherein a part of Ni is replaced with elements such as Mn and N which, like Ni, can produce and retain an austenitic phase. These types of steels contain 3.5 to 5.5% and 4 to 6% of Ni, respectively; therefore, these types of stainless steels contain relatively high Ni contents and are inferior in press forming, because the ratio of the ingredients of these AISI types 20] and 202 such as C, Si, Mn, Cr and Ni are calculated based on the view point of the inhibition from the formation of fi-ferrite phase, thus the austenitic phase produced by annealing being highly stable against martensitic transformation. Accordingly, these types of steels lose some of the desirable characteristics of austenitic stainless steels, stretch-formability and stretch-deep drawing which are based on the high, work-hardening property as a result of a partial transformation of austenitic phase into martensitic phase in the forming process. Further, AISI types 201 and 202 products have an excellent hardness and they are preferred for use in parts of structural products, but undesirable for press forming due to the increased springback and occurrence of wrinkle often observed on some products.

TENELON containing a considerably large amount of N is another example of the 18% Cr austenitic stainless steels containing l5% of Mn, 0.7% of N and free of Ni. This steel, however, requires an unusual steelmaking technique and an elevated pressure of the gaseous atmosphere at the time of the melting process due to the inevitability high N content. Besides, its high yield strength presents several problems in subsequent processing, making it unsuitable for press forming and other practical use. Further, a Ni-free steel containing approximately l8% of Cr and as much N as is solid soluble under an atmospheric pressure, will form a double-phase stainless steels consisting of austenitic phase and B-phase. Thus, the desirable property of austenitic stainless steels will be lost.

Still other Ni-free austenitic stainless steels have been described in US. Pat. Nos. 2,862,8l2 and 3,075,839. The stainless steels, according to said two US Patents, contain ll to 14% of Mn, 0.15 to 0.55% of N and a small amount of Cu which can produce austenitic phase. These steels, as the above-mentioned TE- NELON, cause some problems due to the high N and Mn contents. That is to say, the factors which can control the solidsolubility of N include the composition of the molten steel, temperature of molten steel, ingot dimensions and the amount of H being present with N in the steel. Above all, the large amount of H will sharply control the solidsolubility of N. The high N content may produce blow-holes by the interaction between N and H even in austenitic stainless steels, and the extremely high N content may cause the bleeding phenomenon of ingot. Usually, in high-frequency induction melting with 10 kg to 30 kg of ingot, the amount of H in molten steel is approximately 3 ppm and the solidsolubility of N is approximately 0.20% in the said amount of H, as shown in FIG. 2. In the case of the above-mentioned small ingot, a part of the products obtained may satisfy the essential properties as the aus tenitic stainless steels, even if the N content over the allowable limit will produce small blow-holes in the ingot. While in the case of producing steels in a largescale, electric furnace, 6 to 8 ppm of H is the inevitable content in the said steels. Accordingly, an allowable limit ofthe N solubility becomes 0.13 to 0.15% and the excessive amount of N will produce blowholes or bleeding of ingot. Therefore, sound slab cannot be obtained in this case.

Further, the high N content will improve hot working resistance, decrease plasticity at high temperature, and cause cracks around the edges at the time of hot rolling. Accordingly, the N content should be kept as low as possible only if austenitic phase can be obtained.

Also, Mn is the inevitable element for the maintenance of the austenitic phase. However, an excessive amount of Mn will require much time in order to retain the molten steel in steel making and inevitably increase the H content, thereby causing the above-mentioned disadvantages. Besides, the excessive amount will accelerate the oxidation at high temperature under hot working and annealing and cause substantial reduction in the surface quality of finally obtained products. Accordingly, the Mn content should be kept within the lowest possible range if the austenitic phase is stable.

The Cu content which can produce austenitic phase is to be kept within a range from l to 4%, especially, 2 to 4%. A Cu content of 2 to 4% will remarkably cause improvement in corrosion resistance, softening effect and reduction in the amount of Mn and N, since a proper amount of added Cu makes it possible to stabilize the austenitic phase against martensitic transformation 1.5 times the corresponding amount of Mn.

As a result of our continuous study of Nifree stainless steels, it has been made possible for us to easily produce fully austenitic phase stainless steels by use of Mn, N, C and Cr which can produce austenitic phase, thereby reducing the N content to a range at which sound ingot can be obtained in a conventional steelmaking process by means of a large electric furnace, reducing the Cr content of 16% or less and thus eliminating the disadvantages due to the high N content. We discovered that there exists a definite composition region equivalent or superior to AlSl type 304 in corrosion resistance and suitable for use in press forming such as stretch-formability and stretch-deep drawing.

That is to say, the stainless steels of the present invention are the austenitic stainless steels consisting essentially of 0.06 to 0.15% of C, 13 to 16% of Cr, 0.3 to 1.0% of Si, 7.0 to 12.0% ofMn, 0.05 to 0.15% ofN, 1.0 to 4.0% of Cu and the balance of Fe and the other incidental ingredients. They have a fully austenitic phase without any d-phase in the annealed condition and may produce some martensite by deformation or remain the fully austenitic phase after it. In addition, they have extremely improved mechanical properties, workability and corrosion resistance.

In the case of the stainless steels of the present invention, the increase in the C content corresponds to the decrease in the respective Mn, N and Cu contents. However, it is desirable to keep the C content up to 0.15%, since excessive amounts of C will precipitate chrome carbide and reduce the intergranular corrosion resistance.

The Cr content should be kept up to 16% because excessive amounts of Cr cannot produce the fully austenitic phase that is the essential feature of the present invention. On the other hand, it must be kept 13% or more since smaller amounts will decrease corrosion resistance.

So far as corrosion resistance is concerned, the higher Si content is preferable, but it should be kept up to 1.0% since an excessive amount of Si will cause the formation of S-ferrite phase and reduction in hot workability.

The Mn content should be kept to 7% or more since lower amounts cannot retain a fully austenitic phase. It is also desirable to keep the Mn content up to 12% since a higher Mn content will increase the solubility of N, cause blow-holes in the steels due to the increase in H in steel-making, and accelerate the oxidation of the steels at high temperature under hot working and annealing.

The increase in the N content corresponds to the decrease in the C, Mn and Cu contents, but the excessive amount of N will raise initial yield strength indicated by yield stress and hardness and make the steels unsuitable for press forming. Further, the high N content will cause blow-holes at the time of ingot making by the interaction with H in molten steel, and the extremely high N content will cause bleeding of ingot. According to the conventional electric steel-making process, 6-8 ppm of the H content is inevitable in the steels obtained. Thus, in order to produce sound ingot under the condition of the above-mentioned content of H, the N solubility should be kept to 0.13 to 0.15%, or at most to 0.2%. From said reasons the N content should be kept up to 0.15% and the preferable range of the N content is from 0.08 to 0.13%.

The higher Cu content is effective in corrosion resistance and softening effect and makes it possible to decrease the contents of Mn, N and C which can produce austenitic phase, since Cu is more effective than Mn to stabilize the austenitic phase. However, increased amounts of Cu will cause redshortness of the copper and adversely affect hot workability, thus causing fine cracks. 1n the stainless steels of the present invention, it has been possible to remove this disadvantage by using a certain antioxidant. Although the antioxidant used here is expensive, it will improve yield of coilcracks grinding, facilitate steel-making process, and decrease the loss by oxidation. Considering the above ad vantages, use of the antioxidant becomes substantially advantageous. From the above-mentioned reasons, the Cu content should be kept from 1.0 to 4.0%.

Of course, 0.05-0.5% of Mo should be added in order to sharply improve the corrosion resistance of the rare-earth elements; up to 0.1% of Ti, up to 0.005% of B and up to 0.1% of Nb, which are conventionally used as the so-called additional metals, may be added as the incidental ingredients to improve hot workability or some other characteristics; and a trace amount of Ni may be inevitably incorporated from the raw materials. The stainless steels, according to the present invention, will be explained in detail by the accompanying drawings and Tables 1 to 3. All percentages used are by weight unless otherwise specifically indicated.

Since Cu has a softening effect and an effect to facilitate solubility of interstitial elements such as C and N with large austenitic formability, even if these interstitial elements are used excessively, the softening effect can reduce yield strength of steels. This softening effect is explained by the examples N4, N6, N8, N15, H35 and H37 in FIG. 1. FIG. 1 shows the softening effect of Cu in the stainless steels of the present invention, wherein the abscissa indicates true strain and the ordinate true stress. The drawing, wherein the steels N4, N6, N8, N15, H35 and H37 are illustrated shows that the higher Cu content improves softening effect. The softening effect is particularly important in that there is no need for changing the capacity of the conventional press machine when the steels of the present invention are used for press forming and in that there is uniform strain and small springback. Further, FIG. 1 illustrates that the lower N content decreases the yield strength.

FIG. 2 shows sound ingot produced by using 30 kg highfrequency furnace and 6.5 tons of electric furnace. The values of H in the case of using the former are the ones after blowing-in of vapor. As seen in FIG. 2, it is understood that the N content should be kept up to 0.15% for the purpose of getting sound ingot in the industrial scale. It is also understood that it is industrially unrealistic to produce sound ingot containing 0.25 to 0.3% of N, as shown in the afore-mentioned U.S. Pa-

tents.

Table 1 illustrates some examples of chemical compositions, the amount of martensite after 40% tensile deformation for the steels of the present invention, the referenced steels, and the conventional steels. As seen in these examples, the steels of the present invention contain Cu, Cr, Mn, N and C essentially, and have a fully austenitic phase in the annealed condition. They contain up to 0.15% of N in order to obtain sound ingot. They include the metastable and stable austenitic stainless steels. the former being those in which a part of the fully austenitic phase has been transformed into martensite after deformation and the latter being those in which all the fully austenitic phase remains untransformed even after deformation.

Table 1 Examples of Chemical Compositions and Amounts of Martensite after 40% of Tensile Deformation for the Steels According to the Present Invention, Referenced Steels and Conventional Steels.

Type Chemical Composition (91:) Amount of Description Number Grade Designation C Si Mn Cr Cu N Ni Martensite H 32 15 Cr-8 Mn-2 Cu 0.11 0.52 8.24 15.13 2.09 0.14 9.2 H 34 15 Cr-8 Mn-2 Cu 0.12 0.51 8.11 15.30 2.05 0.08 20.0 Steels of H 33 15 Cr-S Mn-3 Cu 0.11 0.54 7.55 15.17 2.95 0.15 t r the Present H 35 15 Cr-8 Mn-3 Cu 0.11 0.56 8.06 15.20 2.87 0.07 9.0 H 37 15 Cr-10 Mn-2 Cu 0.10 0.44 10.19 14.82 1.79 0.14 tr Invention H 46 Cr-10 Mn-2 Cu 0.09 0.50 10.01 15.08 2.14 0.10 5.7 H 39 15 Cr-lO Mn-2 Cu 0.12 0.39 9.67 15.30 2.05 0.08 7.4 H 38 15 Cr-l0 Mn-3 Cu 0.11 0.43 10.19 15.24 3.35 0.15 t r N 2 15 Cr-13 Mn 0.12 0.52 12.80 14.69 0.19 3.7 Referenced N 4 14 Cr-14 Mn 0.13 0.56 14.20 14.08 0.18 2.0 N 6 13 Cr-17 Mn 0.06 0.48 17.00 12.80 0.19 2.2 Steels N 8 14 Cr-14 Mn 0.13 0.44 14.20 14.08 0.27 t r N 15 14 Cr-14 Mn-2 Cu 0.13 0.48 13.40 14.13 1.98 0.19 t r N 18 1S Cr-13 Mn-l Cu 0.10 0.42 12.80 14.92 0.89 0.26 tr A181 430 17 Cr 0.07 0.47 0.28 16.60 0.03 Conventional A181 301 17 Cr-7 Ni 0.11 0.57 0.99 17.20 0.01 7.58 46.0

A151 304 18 Cr-8 Ni 0.08 0.59 1.06 18.38 0.01 8.91 t r Steels A151 201 17 Cr-6.5 Mn-4.5 Ni 0.10 0.43 6.61 17.13 0.14 4.57 t r A151 202 18 Cr-9 Mn-5.5 Ni 0.07 0.51 9.13 17.92 0.14 5.59 t r N otes:

All examples are tested with sound ingots and melted by kg high-frequency furnace.

In Type Numbers N 8 and N 18. blow-holes occur just below the ingot head.

Table 2 illustrates some mechanical properties and formabilities of the steels of the present invention, the referenced steels, and the conventional steels. From these values, it is clearly understood that the steels of the present invention are equivalent to AlSl types 201, 202, 301 and 304 in mechanical properties, superior to AlSl types 201 and 202, and equivalent to AlSl types 301 and 302 in formability. Particularly in the stainless Table 2 Mechanical Properties and Formability Test Results for the Steels According to the Present Invention, Referenced Steels and Conventional Steels (Sample Thickness 0.8 mmt) Tensile Test Formability Test Spring Type Hardness Yield Tensile Elong- Conical Erichsen Back Description Number Grade Designation Strength Strength gation cup Value Value Angle" Hv( 10) (kg/mm*) (kg/mm) (9%) H 32 15Cr-8Mn-2Cu 177 36 79 24.1 13.6 Steels of H 34 15Cr 8Mn-2Cu 178 31 91 57 23.7 13.4

H 33 15Cr-8Mn-3Cu 170 36 72 56 24.2 13.3 the Present H 35 15Cr-8Mn-3Cu 30 73 57 24.6 14.3 2.7 Invention H 37 15Cr-10Mn-2Cu 176 37 72 59 23.2 12.7 H 46 15Cr-10Mn-2Cu 153 31 74 58 24.0 14.0 2.8 H 39 15Cr-10Mn-2Cu 177 32 74 59 23.6 13.5 H 38 15Cr-10Mn-3Cu 33 67 57 23.6 12.6

N 2 15Cr-13Mn 226 42 87 56 24.4 13.2 Referenced N 4 14Cr-l4Mn 196 41 84 54 24.4 13.1 4.8 N 6 13Cr-17Mn 182 39 80 52 24.2 12.3 5.5 Steels N 8 l4Cr-l4Mn 239 48 84 53 23.0 11.9 4.5 N 15 l4Cr-l4Mn-2Cu 38 74 57 23.8 11.8 2 8 N 18 15Cr-13Mn-1Cu 230 46 81 55 23.5 11.8

A1Sl430 17Cr 160 38 55 30 19.8 9.2 Conventional A1Sl301 l7Cr-7Ni 160 28 81 61 24.4 14.6 3.1 Steels M81304 l8Cr-8Ni 160 30 66 58 24.0 12.1 3.0 AlS1201 17Cr-6.5Mn-4.5N1 197 36 76 59 23.5 1 1.9 AlS1202 18Cr-9Mn-5.5Ni 176 35 70 58 23.8 1 1.9

"Spring back angle: Spring ack angle after releasing the right angle test plate.

with more excellent corrosion resistance than the conventional steels.

Table 3 Corrosion Resistance Tests for the Steels According to the Present Invention, Referenced Steels and Conventional Steels Type Number Description Number Grade Designation of Pit H 32 15Cr8Mn-2Cu 29 Steels of H 34 lSCr-BMn'ZCu 42 H 33 15Cr-8Mn-3Cu 2 H 35 l5Cr-8Mn-3Cu 5 the Present H 37 Cr-l0Mn-2Cu 19 H 46 15Cr-l0Mn-2Cu 24 lnvention H 39 15Cr-10Mn-2Cu 37 H 38 15Cr-10Mn-3Cu 2 N 2 15Cr-13Mn 154 Referenced N 4 14Cr-14Mn 162 N 6 13Cr-17Mn 141 N 8 14Cr14Mn 103 Steels N 15 14Cr-14Mn2Cu 18 N 18 l5Cr-13Mn-1Cu 67 A181 430 17C: 131 Conventional A181 301 17Cr-7Ni 45 A151 304 18Cr-8Ni 33 Steels M51 201 l7Cr-6.5Mn-4.5Ni 69 AIS! 202 18Cr-9-Mn-55Ni 55 A solusodium sulfate 0.5g

lion presodium sulfite 0.253 with a pared by sodium thiosulfate 0.1g solution mixing a sodium chloride 525g consistsolution water 525cc ing of consisting of {calcium chloride 52.5g Rotation dipping water 525cc for 100 hours Number of Fit observed on a sample of 50 mmXl 10 mm According to the present invention and as described above, austenitic stainless steels free of Ni can be obtained which are remarkably excellent in corrosion resistance, in the presence of such N content as to produce sound ingot by a conventional electric steelmaking process. Further, it has been found possible to produce the metastable or stable austenitic stainless steels by balancing their ingredients according to the present invention. Thus, the stainless steels of the present invention are quite advantageous and useful for the commercial scale production in view of the inexpensive raw materials used.

What is claimed is:

1. Substantially nickel free austenitic stainless steels with excellent formability and corrosion resistance, consisting essentially of0.06 to 0.15% of C, 0.3 to 1.0% of Si, 13 to 16% of Cr, 7.55 to 10.19% of Mn, 0.05 to 0.15% of N, and 1.0 to 4.0% of Cu and the balance of Fe as the essential ingredients.

2. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is at least about 0.07% N.

3. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is within the range of about 0.07% to 0.14%.

4. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said Mn content is at least about 8.06%.

5. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is no more than 0.14%.

6. Substantially nickel free austenitic stainless steels with excellent formability and corrosion resistance, consisting essentially of 0.09 to 0.12% of C, 0.39 to 0.56% of Si, 14.82 to 15.30% of Cr, 7.55 to 10.19% of Mn, 0.07 to 0.15% of N, and 1.79 to 3.35% of Cu and the balance of Fe as the essential ingredients.

7. Substantially nickel free austenitic stainless steels as defined in claim 6, in which the Mn content is within the range of 8.06 to 10.19%, and the Cu content is within the range of 1.79 to 2.87%. 

1. SUBSTANTIALLY NICKEL FREE AUSTENITIC STAINLESS STEELS WITH EXCELLENT FORMABILITY AND CORROSION RESISTANCE, CONSISTING ESSENTIALLY OF 0.06 TO 0.15% OF C, 1.0% OF SI. 13 TO 16% OF CR, 7.55 TO
 10. 19% OF MN, 0.05 TO 0.15% OF N AND 1.0 TO 4.0% OF CU AND THE BALANCE OF FE AS THE ESSENTIAL INGREDIENTS.
 2. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is at least about 0.07% N.
 3. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is within the range of about 0.07% to 0.14%.
 4. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said Mn content is at least about 8.06%.
 5. Substantially nickel free austenitic stainless steels as defined in claim 1, in which said N content is no more than 0.14%.
 6. Substantially nickel free austenitic stainless steels with excellent formability and corrosion resistance, consisting essentially of 0.09 to 0.12% of C, 0.39 to 0.56% of Si, 14.82 to 15.30% of Cr, 7.55 to 10.19% of Mn, 0.07 to 0.15% of N, and 1.79 to 3.35% of Cu and the balance of Fe as the essential ingredients.
 7. Substantially nickel free austenitic stainless steels as defined in claim 6, in which the Mn content is within the range of 8.06 to 10.19%, and the Cu content is within the range of 1.79 to 2.87%. 